Test::SMTP is a subclass of Net::SMTP_auth, that is a subclass of Net::SMTP, that in turn is a subclass of Net::Cmd and IO::Socket::INET. Don't be too confident of it beeing a Net::SMTP_auth subclass for too much time, though (v 0.03 changed from Net::SMTP to Net::SMTP_auth so you can control authentication tests better). Compatibility will always try to be kept so you can still call the subclass methods.